The goal of this research is to understand the mechanisms whereby the Tat protein and TAR element of human immunodeficiency virus (HIV-1) affect viral gene expression, and how these moieties contribute to viral pathogenicity. Tat interacts with transcripts of the TAR region to transactivate expression of genes linked to the HIV-1 long terminal repeat (LTR). In an adenovirus-derived model system, we have shown that this transactivation is bi-modal: transcriptional initiation and the processivity of RNA polymerase are both increased. The elongation effect predominates when the basal rate of transcriptional initiation at the HIV-1 LTR is elevated, either by general transactivators with which Tat synergizes in the adenovirus system, or by other genetic elements such as the SV40 replication origin in a transient expression system employing replicating plasmids. We propose to extend these findings to cell types that reflect the normal hosts for HIV-1 infection, as well as proviral cell lines, and to analyze the mechanism of transactivation by Tat in greater depth. Mechanistic studies will be pursued in vivo and in a cell-free transcription system in vitro. We will explore the roles played in Tat-transactivation by LTR elements other than TAR, and investigate the influence of the SV40 origin of DNA replication on transcription from the HIV LTR. Finally, we will address the possibility that transcripts of the TAR region affect translational efficiency and will examine the mechanisms underlying this effect.